For fish living in the murky waters of the Amazon basin, it might seem hard to find a mate. But several species of knife fish have come up with an ingenious solution: In order to rendezvous for nocturnal trysts, they signal to each other with gentle electric shocks. Some use a single burst of direct current, but most have developed a more complex signal, consisting of a pulse of positive voltage followed by a negative one. In today's Nature, biologists explain why the fish prefer this "biphasic" signal: It keeps their love messages from being intercepted by predators.
Electric fish first developed their unique abilities as a means of locating prey in the dark, says neurobiologist Philip Stoddard of Florida International University. Like an electrocardiograph, they detect the telltale pulses of electricity caused by the twitching of another fish's muscle. Later, they evolved an electric organ--a row of battery-like cells under their skin that produces an electric field. By generating their own electric signals, they could navigate (in the same way bats use sonar) and communicate with their own kind. The drawback: Other fish sensitive to electricity, such as electric eels and catfish, could detect and eat them.
Stoddard suspected that biphasic signals act as a sort of cloak of invisibility. He guessed that when a quick positive pulse is followed by a quick negative, they cancel out and a hungry electric eel doesn't notice a thing. To prove this, Stoddard trained an eel to swim toward an electrode when it detected a signal coming from it. "He'd show up expecting food way more often with a monophasic signal, even though it's only half the amplitude of the biphasic signal," Stoddard says. To pick up a potential mate's signal, the knife fish themselves have evolved a secondary electrical receptor that detects high-frequency electric oscillations as well as low-frequency ones.
"This demonstrates an unexpected turnaround in the way we think about the evolution of animal signaling," says Stephen Nowicki, a behavioral neurobiologist at Duke University. Usually, Nowicki explains, the harsh reality of predation limits animals' tendencies to develop complex signals or showy displays as a means of impressing mates. "Just the opposite seems to be the case with the electric fish," says Nowicki: There, predation only made the signaling system more complex.